This project deals with investigations into the regulation and function of the MAPK Kinase Kinase MEKK3. MAPK signaling pathways have been shown to be essential for cell growth, differentiation and apoptosis. It is therefore important to gain a better molecular understanding of the component parts that make up MAPK signaling paths, and of the physiologic targets of individual MAPK pathways in normal and diseased states. We originally cloned MEKK3 and determined that it is capable of activating all four major MAPKs, namely ERK, JNK, p38 and ERK5, albeit to different extents. When overexpressed, an active form of MEKK3 is able to arrest growth, protect cells from some apoptotic insults and can activate NF-kappaB. MEKK3 has been proposed to have a direct role in activating the IKK kinase complex, the critical complex that phosphorylates and thus causes degradation of the inhibitors of NF-kappaB. We have cloned an adaptor protein that directly links MEKK3 with the IKK kinase complex and we have determined that this adaptor, when overexpressed, negatively regulates TNF-mediated activation of the MAPK JNK/SAPK. This gene for this adaptor protein was recently found mutated in about 13% of all patients with inherited Cerebral Cavernous Malformations (CCMs). CCMs are congenital vascular anomalies of the CNS characterized by grossly enlarged blood vessels that can cause hemorraghic strokes, seizures, recurrent headaches and focal neurologic deficits. To explore the in vivo roles of MEKK3 and its adaptor in health and in CCMs we have now generated mice lacking the adaptor on one allele and we have made further progress in generating mice conditionally deficient in MEKK3.